This article is from the Dr. Pepper FAQ, by Christopher Flaherty with numerous contributions by others.
Hot Dr Pepper? Yes, indeed. It's a real drink, and it's been around
for quite a while--at least since the early 1960's (even though
there's no mention of it at all in the 1965 edition of Cookin' With Dr
Pepper--go figure). However, since Dr Pepper--and the soft drink
industry in the U.S. as a whole--switched from cane sugar to high
fructose corn syrup as a sweetener, many people have complained that
Hot Dr Pepper does not taste nearly as good as it did before the
switch, so the Dr Pepper company has refrained from promoting the
recipe as ardently as it had in the past. Nevertheless, people still
drink it hot, with the recommendation that Dr Pepper with cane sugar
be used for best results (see question 12 for how to get some cane
sugar Dr Pepper for yourself).
The recipe itself is quite simple: First, cut a nice, thin, round
slice of lemon for yourself and place it in the bottom of a cup--I
suppose a coffee mug or teacup works best. Next, heat your Dr Pepper
in a saucepan until it looks like it's boiling (even though it'll only
be about 180° F, the carbonation will make it look hotter). Then pour
your "steaming" hot Dr Pepper into the cup, over the lemon slice.
That's it! I haven't tried it yet myself, but people who have swear
it's pretty good.
(There is an official recipe on the www.drpepper.com web site, but
it's not too much different from the one above. Then again, how many
different ways can you say "Heat Dr Pepper and pour it over lemon"?)